Statistical analysis and comparative study of multi-scale 2D and 3D shape features for unbound granular geomaterials

Morphological analysis is a prerequisite step for studying the mechanical properties of unbound granular geomaterials through both laboratory experiments and numerical simulations. In this study, two typical geomaterials, e.g., ballast and cobble particles, were sampled for study. First, a systematic framework was established to acquire two-dimensional (2D) particle outline and three-dimensional (3D) morphology for statistical shape analysis. The photographic-based system was set up to acquire 2D images of particle projections from multiple surrounding directions, then the images obtained were used to reconstruct 3D particle morphology based on close-range photogrammetry. Second, in order to quantify particle shape at three different geometric scales, 6 well-acknowledged 2D and 3D geometry descriptors, i.e., aspect ratio, sphericity, convexity, roundness, regularity, and roughness were computed. Third, the probability distribution function (PDF) of shape descriptors were fitted with different functions. Finally, 2D and 3D shape descriptors were statistically compared and the relationships between them were revealed by analyzing their cumulative distribution functions (CDFs).